MDL-10905 iterating correctly with readdir (as shown in php doc)

mod/assignment/db/postgres7.php

@@ -60,7 +60,7 @@ function assignment_upgrade($oldversion) {
         notify("Moving location of assignment files...");
 
         $basedir = opendir("$CFG->dataroot");
-        while ($dir = readdir($basedir)) {
+        while (false !== ($dir = readdir($basedir))) {
             if ($dir == "." || $dir == ".." || $dir == "users") {
                 continue;
             }